- Haskell-Cafe - Haskell.org

Creating Repa arrays from unboxed vectors
by Janek S. 04 Oct '12

04 Oct '12

Dear list, I'm trying to create unboxed REPA array from unboxed Vector, but I keep getting this type error: ghci> :m + Data.Array.Repa ghci> :m + Data.Array.Repa.Repr.Unboxed ghci> :m + Data.Vector.Unboxed ghci> fromUnboxed Z (Data.Vector.Unboxed.singleton 1) <interactive>:5:16: Couldn't match expected type `vector-0.9.1:Data.Vector.Unboxed.Base.Vector e0' with actual type `Data.Vector.Unboxed.Vector a0' In the return type of a call of `Data.Vector.Unboxed.singleton' In the second argument of `fromUnboxed', namely `(Data.Vector.Unboxed.singleton 1)' In the expression: fromUnboxed Z (Data.Vector.Unboxed.singleton 1) I am confused, because REPA documentation lists Vector from Data.Vector.Unboxed package as its second parameter. Why am I getting this error? I have Repa 3.2 an vector 0.9.1. Janek

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Problems installing DPH
by Janek S. 03 Oct '12

03 Oct '12

Dear list, I ran into problems when installing DPH examples: [root@GLaDOS : ~] cabal --global install dph-examples Resolving dependencies... Configuring dph-examples-0.6.1.3... Building dph-examples-0.6.1.3... Preprocessing executable 'dph-spectral-smvm' for dph-examples-0.6.1.3... [1 of 3] Compiling Vectorised ( examples/spectral/SMVM/dph/Vectorised.hs, dist/build/dph-spectral-smvm/dph-spectral-smvm-tmp/Vectorised.o ) exprType TYPE ghc-prim:GHC.Types.Double{(w) tc 3u} exprType TYPE ghc-prim:GHC.Types.Double{(w) tc 3u} exprType TYPE ghc-prim:GHC.Types.Double{(w) tc 3u} exprType TYPE ghc-prim:GHC.Types.Double{(w) tc 3u} exprType TYPE ghc-prim:GHC.Types.Double{(w) tc 3u} exprType TYPE (ghc-prim:GHC.Types.Int{(w) tc 3J}, ghc-prim:GHC.Types.Double{(w) tc 3u}) opt: Bitcode stream should be a multiple of 4 bytes in length cabal: Error: some packages failed to install: dph-examples-0.6.1.3 failed during the building phase. The exception was: ExitFailure 1 Gogling the error message leads to various LLVM pages, which suggests that LLVM is the problem. I'm using Debian Squeeze with LLVM 2.6 and I'm thinking that maybe DPH needs newer version. What is the minimum required version of LLVM for DPH to work? Or is this problem caused by semthing else? Janek

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Haskell Weekly News: Issue 246
by Daniel Santa Cruz 03 Oct '12

03 Oct '12

Welcome to issue 246 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of September 23 to September 23, 2012. Want to contribute quotes for HWN? Until today the only way to help with those was to use @remember in the IRC channel. Now you can also use @HWNQuotes in twitter to submit quotes for the weekly newsletter. Quotes of the Week * carpi: all said ... the brain of someone of who writes compilers for haskell scares me nuts. I wouldn't want them concentrating on me, lest I catch fire. * Igloo: The major problem, as far as I can see, is how to construct the mumble'. * edwardk: i want to be able to know if the compiler is being stupid or if its just me, and without any sort of model to reason about it with, i always have to assume it could be the compiler; i already have to deal with that problem, its called 'scala' * ddarius: Now I can just point people at a readable and relevant paper instead of having to Cale it. * rwbarton: also, a generally safe answer to "was there an update to lambdabot recently" is "no" * camccann: you want a debugger? GHC has one. Except that stepping through lazy evaluation is like listening to a stream of consciousness narrative told by a 5-year-old with ADHD. Top Reddit Stories * My Haskell book is not dead! I'd love some feedback now that it's six chapters long and growing. Domain: sthaskell.com, Score: 61, Comments: 18 On Reddit: [1] http://goo.gl/AfLwS Original: [2] http://goo.gl/oB0e8 * What commercial users say they want from Haskell Domain: fpcomplete.com, Score: 46, Comments: 54 On Reddit: [3] http://goo.gl/JTtMG Original: [4] http://goo.gl/ZoHrh * Haskell rewarding moments: Orc Battle translation Domain: self.haskell, Score: 41, Comments: 22 On Reddit: [5] http://goo.gl/5xjJJ Original: [6] http://goo.gl/5xjJJ * Does Stroustroup have some FP envy? This is a pretty neat library implementation of pattern matching in C++. Domain: open-std.org, Score: 40, Comments: 44 On Reddit: [7] http://goo.gl/PH7ZU Original: [8] http://goo.gl/KuGrw * Functional TDD: a clash of cultures? Domain: facebook.com, Score: 34, Comments: 34 On Reddit: [9] http://goo.gl/9xBLo Original: [10] http://goo.gl/kOi2G * Dovetailing Domain: self.haskell, Score: 33, Comments: 21 On Reddit: [11] http://goo.gl/GdlMi Original: [12] http://goo.gl/GdlMi * Oh the fun of trying to install wxhaskell Domain: self.haskell, Score: 33, Comments: 33 On Reddit: [13] http://goo.gl/4F3Fu Original: [14] http://goo.gl/4F3Fu * Lazy v. Yield: Incremental, Linear Pretty-printing (Kiselyov, Peyton-Jones, Sabry) [pdf] Domain: cs.indiana.edu, Score: 31, Comments: 54 On Reddit: [15] http://goo.gl/PfVHl Original: [16] http://goo.gl/ub46l * Call for discussion: OverloadedLists extension Domain: gist.github.com, Score: 24, Comments: 52 On Reddit: [17] http://goo.gl/0O1G8 Original: [18] http://goo.gl/xYyTq * Real World Haskell: is there a second edition coming? Domain: self.haskell, Score: 24, Comments: 9 On Reddit: [19] http://goo.gl/rjjQb Original: [20] http://goo.gl/rjjQb * Introduction to Functional Reactive Programming [video] Domain: blog.edwardamsden.com, Score: 23, Comments: 3 On Reddit: [21] http://goo.gl/vSVKP Original: [22] http://goo.gl/IH3MF Top StackOverflow Questions * What optimizations can GHC be expected to perform reliably? votes: 27, answers: 2 Read on SO: [23] http://goo.gl/E6bH4 * A simple example showing that IO doesn't satisfy the monad laws? votes: 22, answers: 4 Read on SO: [24] http://goo.gl/YptHT * Are there any good use cases for OverlappingInstances? votes: 12, answers: 3 Read on SO: [25] http://goo.gl/FLChO * Why is ListT monad transformer considered buggy - what monad laws it breaks? votes: 11, answers: 1 Read on SO: [26] http://goo.gl/HX7Oa * How to build and use a bootstrap executable in a Cabal package votes: 10, answers: 1 Read on SO: [27] http://goo.gl/tvA03 * Continuation passing style representation of types votes: 10, answers: 1 Read on SO: [28] http://goo.gl/6R60n * GHC code generation for type class function calls votes: 10, answers: 3 Read on SO: [29] http://goo.gl/3ggvE * Why inductive datatypes forbid types like `data Bad a = C (Bad a -> a)` where the type recursion occurs in front of ->? votes: 10, answers: 2 Read on SO: [30] http://goo.gl/jtG6d * Are functions of arity-n really just an n-category due to currying? Can they be made into a 1-category? votes: 9, answers: 2 Read on SO: [31] http://goo.gl/JfJLF * Setting socket options on OSX votes: 9, answers: 0 Read on SO: [32] http://goo.gl/aybHY Until next time, Daniel Santa Cruz References 1. http://www.sthaskell.com/ 2. http://www.reddit.com/r/haskell/comments/10fx9w/my_haskell_book_is_not_dead… 3. http://fpcomplete.com/commercialuserneeds/ 4. http://www.reddit.com/r/haskell/comments/10me87/what_commercial_users_say_t… 5. http://www.reddit.com/r/haskell/comments/10c7n5/haskell_rewarding_moments_o… 6. http://www.reddit.com/r/haskell/comments/10c7n5/haskell_rewarding_moments_o… 7. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3449.pdf 8. http://www.reddit.com/r/haskell/comments/10i859/does_stroustroup_have_some_… 9. https://www.facebook.com/notes/kent-beck/functional-tdd-a-clash-of-cultures… 10. http://www.reddit.com/r/haskell/comments/10jk06/functional_tdd_a_clash_of_c… 11. http://www.reddit.com/r/haskell/comments/10d7gx/dovetailing/ 12. http://www.reddit.com/r/haskell/comments/10d7gx/dovetailing/ 13. http://www.reddit.com/r/haskell/comments/10lyaw/oh_the_fun_of_trying_to_ins… 14. http://www.reddit.com/r/haskell/comments/10lyaw/oh_the_fun_of_trying_to_ins… 15. http://www.cs.indiana.edu/~sabry/papers/yield-pp.pdf 16. http://www.reddit.com/r/haskell/comments/10nsmw/lazy_v_yield_incremental_li… 17. https://gist.github.com/3761252 18. http://www.reddit.com/r/haskell/comments/10c8ld/call_for_discussion_overloa… 19. http://www.reddit.com/r/haskell/comments/10gcpp/real_world_haskell_is_there… 20. http://www.reddit.com/r/haskell/comments/10gcpp/real_world_haskell_is_there… 21. http://blog.edwardamsden.com/2012/09/video-introduction-to-functional.html 22. http://www.reddit.com/r/haskell/comments/10nho4/introduction_to_functional_… 23. http://stackoverflow.com/questions/12653787/what-optimizations-can-ghc-be-e… 24. http://stackoverflow.com/questions/12617664/a-simple-example-showing-that-i… 25. http://stackoverflow.com/questions/12628700/are-there-any-good-use-cases-fo… 26. http://stackoverflow.com/questions/12617916/why-is-listt-monad-transformer-… 27. http://stackoverflow.com/questions/12571205/how-to-build-and-use-a-bootstra… 28. http://stackoverflow.com/questions/12589765/continuation-passing-style-repr… 29. http://stackoverflow.com/questions/12645254/ghc-code-generation-for-type-cl… 30. http://stackoverflow.com/questions/12651146/why-inductive-datatypes-forbid-… 31. http://stackoverflow.com/questions/12571161/are-functions-of-arity-n-really… 32. http://stackoverflow.com/questions/12590202/setting-socket-options-on-osx

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Re: [Haskell-cafe] Explicitly passing an argument to an arrow
by Alfredo Di Napoli 03 Oct '12

03 Oct '12

I see. There exists an equivalent version but more generic? Just out of curiosity, I'm still pretty new to arrows, as you may have read :) Thanks, A. On 3 October 2012 15:59, Ertugrul Söylemez <es(a)ertes.de> wrote: > Alfredo Di Napoli <alfredo.dinapoli(a)gmail.com> wrote: > > > Solution was simple, after reading that functions are arrows: > > > > swapA' t = (swapFirst >>> swapSecond) (t,t) > > where > > swapFirst = first snd > > swapSecond = second fst > > No, that's not a solution, unless a = (->) is acceptable. This code is > specific to the function arrow. > > > Greets, > Ertugrul > > -- > Key-ID: E5DD8D11 "Ertugrul Soeylemez <es(a)ertes.de>" > FPrint: BD28 3E3F BE63 BADD 4157 9134 D56A 37FA E5DD 8D11 > Keysrv: hkp://subkeys.pgp.net/ >

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Explicitly passing an argument to an arrow
by Alfredo Di Napoli 03 Oct '12

03 Oct '12

Hello everyone, sorry for the dumb question but I'm wrapping my head around arrow just from this morning. Consider this toy function to swap argument of a tuple: swapA' :: (Arrow a) => a ((b,c), (b,c)) (c,b) swapA' = swapFirst >>> swapSecond where swapFirst = first $ arr snd swapSecond = second $ arr fst It works but requires to pass a tuple of tuple, namely ((b,c), (b,c)). How can I explicitly pass my tuple of tuple to swapFirst so I can simply invoke swapA' (1,2) and get the correct result? ps. I know that swap can be easily and elegantly be written as: swap = snd &&& fst but the point of the exercise was to experiment with first, second and arrow concatenation Thanks in advance, Alfredo

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Data.Text and quasi quoting
by Jake Wheat 03 Oct '12

03 Oct '12

Hello, I have a parser which parses to an ast which contains Text values. I am trying to use this parser with quasiquoting, but the implementation of Data for Text is incomplete. I've attached a smallish test case, when I try to compile Text.hs I get: Text.hs:17:9: Exception when trying to run compile-time code: Data.Text.Text.toConstr Code: Language.Haskell.TH.Quote.quoteExp expr " test " Is there a way to get this working? Thanks, Jake.

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Strange things with type literals of kind Nat
by Troels Henriksen 02 Oct '12

02 Oct '12

Consider the following definition of lists with a statically determined minimum length. data Nat' = Z | S Nat' data NList (n::Nat') a where Rest :: [a] -> NList Z a (:>) :: a -> NList n a -> NList (S n) a uncons :: NList (S n) a -> (a, NList n a) uncons (x :> xs) = (x, xs) This works fine. Now consider an implementation using the new type literals in GHC: data NList (n::Nat) a where Rest :: NList 0 [a] (:>) :: a -> NList n a -> NList (n+1) a uncons :: NList (n+1) a -> NList n a uncons (x :> l) = l This gives a type error: /home/athas/code/nonempty_lists.hs:41:19: Could not deduce (n1 ~ n) from the context ((n + 1) ~ (n1 + 1)) bound by a pattern with constructor :> :: forall a (n :: Nat). a -> NList n a -> NList (n + 1) a, in an equation for `uncons' at /home/athas/code/nonempty_lists.hs:41:9-14 `n1' is a rigid type variable bound by a pattern with constructor :> :: forall a (n :: Nat). a -> NList n a -> NList (n + 1) a, in an equation for `uncons' at /home/athas/code/nonempty_lists.hs:41:9 `n' is a rigid type variable bound by the type signature for uncons :: NList (n + 1) a -> NList n a at /home/athas/code/nonempty_lists.hs:40:11 Expected type: NList n a Actual type: NList n1 a In the expression: l In an equation for `uncons': uncons (x :> l) = l I don't understand why GHC cannot infer that the two types are the same. My guess is that 'n+1' is not "structural" to GHC in the same way that 'S n' is. The page http://hackage.haskell.org/trac/ghc/wiki/TypeNats/MatchingOnNats mentions that "type level numbers of kind Nat have no structure", which seems to support my suspicion. What's the complete story, though? -- \ Troels /\ Henriksen

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ANNOUNCE: cmaes, a generic optimizer
by Takayuki Muranushi 02 Oct '12

02 Oct '12

Hello, I'm pleased to announce "cmaes" http://hackage.haskell.org/package/cmaes , a library for optimization (in mathematical sense). That is, given a function f :: a->Double, to find x of type a so that (f x) is as small as possible. CMA-ES is an optimization algorithm developed by Nikolaus Hansen and his folks http://www.lri.fr/~hansen/cmaesintro.html . It is a stochastic approach, it does not require gradient information. These properties makes it robust against noise and makes it widely applicable. The cmaes packages is a Haskell wrapper to their elaborate Python code. The robustness of CMA-ES and generic programming put together, you can now describe optimization problems for various types of functions just in one line. CMA-ES is not a silver bullet, of course, but it's been working fine for many application including mine, and I hope this library will drastically reduce the initial cost of trying one for you. Type will tell what cmaes can do. Please refer to the running document http://hackage.haskell.org/packages/archive/cmaes/0.2.1/doc/html/Numeric-Op… . You can optimize functions of type ([Double]-> Double) and ([Double]-> IO Double) , that's usual. Next up, you can optimize (Traversable t => t Double-> Double) and (Traversable t => t Double-> IO Double) where you are pretty sure about which parameter you're tuning. Finally, you can do (Data a => a -> Double) and (Data a => a -> IO Double) , where SYB will find the hidden Doubles to be optimized over for you. It was my joy to see optimization of function of type such as (Triangle -> Double) works out of the box. Any comments, suggestions, bug reports are welcome! Thanks, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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Copying a syntax tree
by Sergey Mironov 02 Oct '12

02 Oct '12

Hi! I have a syntax tree defined like this: > > data A a = A Int Int (B a) > > data B a = B String String (C a) > > data C a = C Int Int (D a) > and so on, all the data are parametrized with a type variable. This variable is actually used as a field in the very end of a hierarchy: > > data D a = D Int a > Now I have to write a function which would copy (A Int) to (A String). Is it possible to do so using TH/syb without writing copyA (A i1 i2 b) = A i1 i2 (copyB b) copyB = ... copyC = ... .. copyD (D i a) = D i (show a) ? Could you provide me with a hint? Thanks, Sergey

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Possible bug in Criterion or Statistics package
by Till Berger 02 Oct '12

02 Oct '12

Dear all, I may have stumbled upon a bug in the Criterion package. When running the attached Haskell program (Benchmark.hs, a simple test case) on multiple cores (with +RTS -N, +RTS -N2, +RTS -N3 etc.) it sooner or later crashes with the following exception: Benchmark: thread blocked indefinitely in an MVar operation With profiling support enabled and run with the xc flag I get the following output before the crash: *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main, called from Main.CAF --> evaluated by: Main.main, called from Main.CAF *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main, called from Main.CAF I have tested this with GHC versions 7.0.4 and 7.4.2 and Criterion 0.6.0.1. So I am not sure if this is a bug in Criterion itself, the Statistics package or any dependency or if I am doing something obviously wrong. I would be grateful if someone could look into this as it is holding me back from using Criterion for benchmarking my code. Regards, Till Berger

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